A tea extract to be used for the preparation of a packaged tea beverage is conventionally obtained by charging tea leaves and hot water for extraction in an opened extraction tank which is generally called kneader, stirring the mixture and then taking out the extract from the tank. Of the ingredients contained in the tea leaves, non-polymer catechins cannot be extracted so easily as caffeine when the tea leaves are extracted with hot water. According to the conventional method, therefore, a caffeine/non-polymer catechins ratio in the extract inevitably becomes higher than that in the tea leaves, in other words, higher than the natural composition. A caffeine/non-polymer catechins ratio in the extract can be reduced close to that in the tea leaves by extracting under severer conditions such as longer stirring time or higher extraction temperature and thereby increasing an extraction percentage of the non-polymer catechins, but this method has a disadvantage of impairing the flavor and taste of the resulting tea extract.
In order to obtain a tea extract having good flavor and taste, there have been reported a method (Japanese Patent Application Laid-Open No. 2000-50799) of depositing tea leaves in a tea extracting tank equipped with a metal mesh, taking out a tea extract from below the tea leaves while feeding, from above the tea leaves, water of from 0 to 36° C. from which dissolved oxygen has been removed, and removing the precipitate by cooling the extract to from −1 to 6° C.; and a method (Japanese Patent Application Laid-Open No. Hei 6-178651) of adding, to tea leaves, 2 to 3 times the weight, based on the weight of the tea leaves, of water to wet them for several minutes, pouring 2 to 5 times the weight, based on the weight of the tea leaves, of hot water of from 80 to 95° C. to the wetted tea leaves to cause elution of a drinkable ingredient therefrom, and obtaining an eluate having a concentration of from 5 to 10 Brix % as a concentrated tea beverage.
In the method as described in Japanese Patent Application Laid-Open No. 2000-50799, however, an extraction percentage of non-polymer catechins from the tea leaves is low owing to the extraction at low temperature. Production of a packaged tea beverage by using this method is therefore not economically efficient. The method as described in Japanese Patent Application Laid-Open No. Hei 6-178651 uses only a small amount of an extracting liquid in order to prepare a concentrated extract so that an extraction percentage of non-polymer catechins contained in the tea leaves is low and production of a packaged tea beverage by using this method is also economically inefficient. Moreover, a caffeine/non-polymer catechins ratio in the extract becomes higher than that of the tea leaves, that is, natural composition.
A conventional preparation process of a tea extract by using a kneader, that is, an opened extraction tank is on the other hand accompanied with the drawbacks that vapor dissipation from the extraction tank during extraction work deteriorates the working environment and it also deteriorates the quality of the tea extract owing to the dissipation of a flavor ingredient in the tea and oxidation of the tea.
As means for overcoming the above-described problems, there have been reported a process (Japanese Patent Application Laid-Open No. Hei 7-23714) of using a closed extraction tank and obtaining stirring effects by blowing an inert gas from below the extraction tank, and a process (Japanese Patent Laid-Open No. 11-103777) of feeding tea leaves and extracting water in a closed extraction tank, extracting the tea leaves in the extraction tank and taking out the extract from the extraction tank, wherein the extract once taken out from the extraction tank is refluxed in the extraction tank again and then taken out.
These processes need a temperature increase or prolongation of time upon extraction to raise the extraction efficiency, which however deteriorates the flavor and taste of the tea extract thus obtained. In addition, fine powder and coarse taste of ingredients inevitably get mixed in the tea extract owing to the floating of the tea leaves in the extraction tank, which deteriorates the flavor and taste.